Method of stabilizing polyacrylonitrile spinning solutions

ABSTRACT

A PROCESS FOR PREPARING A STABLE SPINNING COMPOSITION COMPRISING AN ACRYLONITRILE POLYMER DISSOLVED IN A CONCENTRATED INORGANIC SOLVENT SOLUTION THEREFOR AND A SURFACE ACTIVE AGENT INSOLUBLE IN SAID SOLUTION IS DISCOLSED WHICH COMPRISES DISPERSING SAID SURFACE ACTIVE AGENT IN SAID SOLUTION, MAINTAINING SAID DISPERSION AS A FILM UNDER RETION, AND DEFOAMING SAID DISPERSION AS A FILM UNDER REDUCED PRESSURE AT A TEMPERATURE WHICH IS AT LEAST EQUAL TO THE BOILING POINT OF SAID COMPOSITION AT THE REDUCED PRESSURE EMPLOYED.

United States Patent 3,630,985 METHOD OF STABILIZING POLYACRYLONITRILE SPINNING SOLUTIONS Kenji Takeya, Okayama, and Toshiyuki Kobashi, Tsukuhogun, Japan, assignors to American Cyanamid Company, Stamford, Conn. N0 Drawing. Filed May 13, 1970, Ser. No. 37,006

Int. Cl. C08f 47 /16 US. Cl. 26029.6 AN 8 Claims ABSTRACT OF THE DISCLOSURE A process for preparing a stable spinning composition comprising an acrylonitrile polymer dissolved in a concentrated inorganic solvent solution therefor and a surface active agent insoluble in said solution is disclosed which comprises dispersing said surface active agent in said solution, maintaining said dispersion under agitation, and defoaming said dispersion as a film under reduced pressure at a temperature which is at least equal to the boiling point of said composition at the reduced pressure employed.

BACKGROUND OF THE INVENTION This invention relates to a method for stabilizing a spinning composition of an acrylonitrile polymer. More particularly, this invention relates to a process for removing foam from a spinning composition comprising an acrylonitrile polymer dissolved in a concentrated inorganic solvent solution therefor, and a surface active agent insoluble in said spinning composition whereby the surface active agent remains stably and uniformly dispersed in said spinning solution.

There have been suggested many methods for improving the properties of acrylic fibers by use of various additives in the spinning solutions. For example, it is known to add various inorganic compounds such as titanium oxide, antimony oxide, inorganic acid sulfites, and sulfuric acid, various organic compounds such as alcohols and carboxylic acids, and various phosphoric acid compounds such as phosphate and phosphite derivatives.

It is also known to add nonionic surface active agents to solutions of polyacrylonitrile in dimethyl formamide to avoid increases in solution viscosity and to avoid coloration of such solution. In such process the added surface active agent is completely soluble in the solution in which it is employed. Because of the complete solubility of added surface active agent in the polymer solution, no problems with respect to separation of surface active agent upon foaming or long term storage are encountered.

In the present instance, however, the polymer solvent is a concentrated aqueous solution of an inorganic salt such as a thiocyanate salt, a metal halide such as zinc chloride, mixed salts or an inorganic acid such as nitric acid. Surface active agents are not soluble in such concentrated aqueous solution and when dispersed therein separate as agglomerates on the surface of the spinning solution, the agglomeration being intensified by dispersed air or foam which develops in the spinning solution. The higher the amount of air dispersed in the spinning solu tion, the greater is the amount of surface active agent agglomerated on the surface of the spinning solution. The amount of agglomerated surface active agent also increases with age of the spinning solution and leads to extreme variations in concentration of surface active agent at various levels of the spinning solution. Such variations in concentration of surface active agent gives rise to problems such as filament breakage and loss of continuous processing during spinning and also leads to losses in tenacity and dyeability of the spun fiber. Thus, the eifect intended by addition of surface active agent cannot be readily attained.

It is also necessary in spinning processes to eliminate foam from spinning solutions to prevent the problem of thread breakage. Two methods of removing foam from the spinning solutions are generally employed. One method involves storing the solution in a tank maintained under reduced pressure. The other method involves instantaneous removal of the foam by exposing the solu-' tion in the form of a thin film to reduced pressure. Neither method can effectively be employed with a spinning solution comprising an acrylonitrile polymer, an inorganic solvent solution therefor, and a dispersed surface active agent. When the tank method is employed, the surface active agent agglomerates around the foam as it rises to the surface of the solution producing a layer on the surface of the spinning solution which is very rich in surface active agent. When the film method is employed, the layer of surface active agent which is already present in the solution has a high viscosity and interferes with proper foam removal. As a consequence, it is impossible to achieve a properly defoamed solution and the problems previously mentioned cannot be avoided.

SUMMARY OF THE INVENTION According to the present invention there is disclosed a process for preparing a stable spinning composition comprising an acrylonitrile polymer dissolved in a con-' centrated inorganic solvent solution for said polymer and a surface active agent insoluble in said solution which comprises dispersing said surface active agent in said solu tion at a particle size less than microns, maintaining said dispersion under agitation, and defoaming said dispersion as a film under reduced pressure at a temperature which is at least equal to the boiling point of said composition at the reduced pressure employed until the foam content is below 0.5%, by volume, based on the total volume of the composition. The spinning composition thus obtained remains stable for extended time periods in the absence of agitation without separation of surface active agent as a surface agglomerate. The content of surface active agent remains uniformly dispersed throughout the composition at the concentration initially added.

Although the temperature at which the film is defoamed is at least equal to the boiling point of the composition at the reduced pressure employed, it is generally preferred that the temperature be in the range given by the expression temperatures being degrees centigrade and the viscosity being in kilograms per meter per second.

In the process of the present invention, it is necessary to achieve a particle size of the dispersed surface active 3 agent below 50 microns in order to produce the desired stable composition.

It is also necessary to maintain the dispersion in a state of agitation until the defoaming step is conducted. Thus, in the storage tank wherein the dispersion is maintained, sufficient agitation is generally obtained when the following conditions are satisfied:

wherein R and N are respectively a Reynolds number and a power number represented by the following formulas:

and N wherein R, is given by the formula wherein d is the pipe diameter, 11 is the average flow velocity through the pipe, p is the density of the dispersion, and is the viscosity of the dispersion.

When the surface active agent is uniformly dispersed at a particle size less than about 50 microns and the properly agitated dispersion is fed as a film into a defoamer which at operating pressure is at at least the boiling point of said dispersion, the dispersion is readily defoamed without agglomeration of the surface active agent. It is necessary that defoaming be carried out so as to reduce any foam content to below 0.5%, preferably 0.1%, by volume, based on the total volume of the dispersion defoamed.

Any of a number of commercially available defoamers which operate with films of the material to be defoamed are suitable for carrying out this step of the process of the present invention. Particularly suitable is a defoamer in which the dispersion flows down the inside wall of a tank maintained at reduced pressure. Preferred reduced pressures are in the range of about 20 to 300 millimeters of mercury absolute.

The term acrylonitrile polymer as used in the present invention means a homopolymer of acrylonitrile or a copolymer of a major portion of acrylonitrile and the balance of one or more monoethylenically unsaturated compounds copolymerizable therewith.

As inorganic solvent solutions for said acrylonitrile polymer may be used a concentrated aqueous solution of an inorganic salt such as sodium, potassium, calcium, or ammonium thiocyan'ate, metal halides such as zinc chloride and those mentioned in US. Pats. 2,648,646 and 2,648,649, mixed salts, nitric acid or sulfuric acid.

The surface active used in the present invention may be added at any stage in the preparation of the polymer solution, that is, before, during, or after dissolution of the acrylonitrile polymer in the inorganic solvent solution. In order to improve fiber properties or for other reasons, other additives may also be incorporated into the spinning dispersion, for example, such additives as titanium oxide, antimony oxide, inorganic acid sulfites, alcohols, carboxylic acids, or phosphorous compounds.

Examples of surface active agents which may be used. in the process of the present invention are included the following: sodium salts of polyoxyethylene cetylsulfates; stearamide polymers; stearamide pyridinium chloride; laurylmethyl pyridinium chloride; quaternary ammonium salts such as lauryldimethylbenzyl ammonium chloride, stearlydimethylbenzyl ammonium chloride, cetyltrimethyl ammonium chloride, hexadecyltrirnethyl ammonium chloride, and octadecyltrimethyl ammonium chloride; laurylamine acetate; long chain alkylethylene ureas such as octadecylethylene urea; sorbitan monolaurate; *alkylketene dimers, higher fatty acid esters of glycerine, such as glyceryl monostearate, and their alkylene oxide adducts; alkyl betaines such as lauryl betaine; polysiloxanes such as dimethyl polysiloxane and methyl hydrogen siloxane; and compounds represented by the formulas onscmo o oand wherein R is an alkyl group. One or more of such surface active agents may be employed. The amount of said surface active agent to be employed is determined by the degree of property improvements desired in the ultimate fibers formed from the spinning solution or other considerations. The amount of said surface active agent will generally fall lWithlD. the use level normally accomplishing the improvements desired, such as from about 0.1 to 10%, 'by weight, based on the weight of polymer in the spinning composition. Preferably the amount of surface active agent is in the range of about 1 to 5%, same basis.

The invention is illustrated by the examples which follow wherein the parts and percentages are by weight unless otherwise specifically indicated.

EXAMPLE 1 A homogeneous solution was prepared by mixing 11 parts of a copolymer of 91% acrylonitrile and 9% methyl acrylate with 89 parts of an aqueous solution of 44% sodium thiocyanate. Next 0.55 part of (5% by weight based on the weight of the polymer) of laurylamine acetate were added to the polymer solution and dispersed to produce a particle size of 10 microns. The dispersion thus formed was kept agitated under conditions wherein the Reynolds number R 800 and the power number N :1.0.

The spinning composition thus obtained was preheated to 70 C. while being maintained under the conditions of agitation specified above and then fed into a falling-film type defoamer of wet wall area of 0.25 square meter. Defoaming was accomplished at a pressure of millimeters of mercury, absolute, with defoamer jacket temperature of 55 C. and an operating speed of 50 kilograms per hour. The foam was reduced to less than 0.01% by volume of air based on the total volume of the spinning dispersion.

The defoamed spinning composition obtained showed no separation of surface active agent and contained 5% of surface active agent uniformly distributed throughout the dispersion. The defoamed dispersion was stored at C. without agitation and without separation of surface active agent even after long storage periods, i.e. 24 hours or more.

Comparative Example A The homogeneous polymer solution of Example 1 was again prepared. Laurylamine acetate was again added in the amounts and in the manner described in Example 1. The resulting dispersion was kept agitated as described in Example 1.

500 liters of the solution thus obtained were fed into a tank of 1200 liters and held at 30 C. at a pressure of 50 millimeters of mercury, absolute, for 25 hours for defoaming. A layer of surface active agent formed in the upper portion of the thus treated dispersion and much foam was contained in this layer. The layer containing surface active agent was far more viscous than the original spinning solution. As a result, even though defoaming was carried out for 25 hours, the foam in the surface active agent layer could not be removed.

When the layer of surface active agent was removed f"om the composition and the balance of the composition analyzed for content of surface active agent at various positions, it was found that the bottom portion contained only 1.4% of surface active agent and the upper portion contained about of surface active agent, the percentages being based on the total weight of polymer in the composition. When the composition was then agitated to become uniform, it was found to contain only 3.1% of surface active agent (based on the weight of polymer), thus indicating a value far below that initially present.

EXAMPLES 2 AND 3 Into a polymer solution prepared in the manner described in Example 1 were added in separate runs 5% on the weight of polymer of glyceryl monostearate adducts of 2 and 6 moles of ethylene oxide. The surface active agents were dispersed to obtain a particle size of 10 microns in each instance. The solutions were maintained in agitated condition and defoamed as in Example 1.

There were obtained spinning compositions in which the air content was less than 0.01% by volume based on the total volume of dispersion and no surface active layer was formed. Analysis of the defoamed compositions indicated the results summarized in Table I below which also identifies the agents employed in each example.

TABLE I 3 Glfi cgr yl monostearate plus 6 moles 1 E.O.=ethylene oxide.

Comparative Examples B and C The dispersions prepared as described in Examples 2 and 3 were defoamed as described in Comparative Example A.

In each instance a layer of surface active agent formed in the upper portion of the dispersion during defoaming and considerable amounts of foam was contained in such layer. The layer could be readily detected by the unaided eye to be rich in content of respective glyceryl monostearate adduct.

The layer of surface active layer was removed and the remaining composition analyzed for content of surface active layer at various positions. The results are given in Table II along with identification of surface active agent.

What is claimed is: 1. A process for stabilizing a spinning composition comprising an acrylonitrile polymer dissolved in a concentrated aqueous inorganic solvent solution for said polymer and a surface active agent insoluble in said solution which process comprises dispersing said surface agent in said solution at a particle size below 50 microns, maintaining said solution under agitation satisfying the conditions given by the expressions 2 e d up and N wherein a is the diameter of the agitating blade, n is the velocity of agitation, gc is the gravity conversion factor, p is the density of the agitated solution and a is the viscosity of the agitated solution, and defoaming said dispersion as a film under reduced pressure at a temperature which is in the range given by the expression wherein T F is the operating temperature in degrees centigrade, T is the boiling point in degrees centigrade of the composition at the operating pressure, and ,u is the viscosity in kilograms per meter mr second of the composition at T to reduce the foam content to less than 0.5%, by volume, based on the total volume of said composition.

2. The process of claim 1 wherein the operating pressure in the defoaming step is in the range of about 20 to 300 millimeters of mercury, absolute.

3. The process of claim 1 wherein the defoaming step is carried out at a pressure of millimeters of mercury, absolute, and a temperature of 55 C.

4. The process of claim 1 wherein the surface active agent is dispersed at a particle size of 10 microns.

5. The process of claim 1 wherein defoaming is carried out to reduce the foam content to below 0.1%, by volume, based on the total volume of said composition.

6. The process of claim 1 wherein said composition contains laurylamine acetate as said surface active agent.

7. The process of claim 1 wherein said composition contains as said surface active agent the adduct of glyceryl monostearate with 2 moles of ethylene oxide.

8. The process of claim 1 wherein said composition contains as said surface active agent the adduct of glyceryl monostearate with 6 moles of ethylene oxide.

References Cited UNITED STATES PATENTS 2,648,649 8/1953 Stanton et al. 260-29.l 3,240,745 3/1966 Cheape, Jr. et al. 26034.2 3,531,557 9/1970 Becker et al 260-34.2

ALLAN LIEBERMAN, Primary Examiner S. L. FOX, Assistant Examiner US. Cl. X.R. 

